Accurate control over the torque applied to threaded fasteners of machinery is of increasing importance in assembly operations. Various devices have been utilized in fastener tightening power tools to shut off the power supply or disengage the tool from the fastener at a predetermined torque output. Some of the more common devices have been torque responsive clutches, pressure-sensing devices which detect a change in inlet or exhaust pressures of the drive motor and rotating spring devices which are activated by reaction to the output torque.
A limitation of these devices, in addition to their inaccuracy and nonrepeatability, is their inability to detect the common fastener failings; for example, cross-threading, thread stripping, chips in the threadhole, defective threads or tool malfunction.
Several methods have been developed which improve torquing accuracy such as the so-called "turn of the nut" or "constant energy application" methods. However, none of these methods has suggested a means for detecting the common fastener faults noted above.